We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation. This general method optimizes the synthesis with initial heating rates up to 100°C∕ minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms, and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these nanocrystals (NCs) into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC's size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.doped nanocrystals | superlattice | lanthanides | luminescence R ecent advances in synthesis and controlled assembly of monodisperse colloidal nanocrystals (NCs) into superlattice structures have enabled their applications in optics (1), electronics (2), magnetic storage (3), etc. Single-and multicomponent superlattices composed of spherical NCs are increasingly studied and a rich family of structures is now accessible (4, 5), where the electronic and magnetic interactions between the constituents gives rise to new cooperative properties (6, 7). New synthetic approaches are yielding nonspherical NCs with physical properties unobtainable by simply tuning the size of the spheres (8-11), providing an even broader array of nanoscale building blocks. The size and shape dependence of NC's biological activity (12, 13) and toxicity (14) is also of intense interest. However, the challenge of precisely controlling particle shape while maintaining uniformity in size and surface functionality has limited studies of NC environmental health and safety just as it has hindered efforts to organize anisotropic building blocks and to establish methods to capture their unique properties in NC superlattice thin films.Lanthanide-doped nanophosphors are an emerging class of optical materials (15). These NCs often possess "peculiar" optical properties [e.g., quantum cutting (16) and photon upconversion (17)], allowing the management of photons that could benefit a variety of areas including biomedical imaging (18, 19) and therapy (20), photovoltaics (16, 21), solid state lighting (22), and display technologies (23). Colloidal upconversion nanophosphors (UCNPs) are capable of converting long-wavelength near-infrared excitation into short-wavelength visible emission through the long-lived, metastable excited states of the lanthanide dopants (24). In contrast to the Stokes-shifted emissions from semiconductor NCs or organic fluorophores and the multiphoton process employing fluorescent dyes, UCNPs offer several ...

When the packing fraction is increased sufficiently, loose particulates jam to form a rigid solid in which the constituents are no longer free to move. In typical granular materials and foams, the thermal energy is too small to produce structural rearrangements. In this zero-temperature (T = 0) limit, multiple diverging and vanishing length scales characterize the approach to a sharp jamming transition. However, because thermal motion becomes relevant when the particles are small enough, it is imperative to understand how these length scales evolve as the temperature is increased. Here we used both colloidal experiments and computer simulations to progress beyond the zero-temperature limit to track one of the key parameters-the overlap distance between neighbouring particles-which vanishes at the T = 0 jamming transition. We find that this structural feature retains a vestige of its T = 0 behaviour and evolves in an unusual manner, which has masked its appearance until now. It is evident as a function of packing fraction at fixed temperature, but not as a function of temperature at fixed packing fraction or pressure. Our results conclusively demonstrate that length scales associated with the T = 0 jamming transition persist in thermal systems, not only in simulations but also in laboratory experiments.

Research on soft materials, including colloidal suspensions, glasses, pastes, emulsions, foams, polymer networks, liquid crystals, granular materials, and cells, has captured the interest of scientists and engineers in fields ranging from physics and chemical engineering to materials science and cell biology. Recent advances in rheological methods to probe mechanical responses of these complex media have been instrumental for producing new understanding of soft matter and for generating novel technological applications. This review surveys these technical developments and current work in the field, with partial aim to illustrate open questions for future research.

We conduct experiments on two-dimensional packings of colloidal thermosensitive hydrogel particles whose packing fraction can be tuned above the jamming transition by varying the temperature. By measuring displacement correlations between particles, we extract the vibrational properties of a corresponding "shadow" system with the same configuration and interactions, but for which the dynamics of the particles are undamped. The vibrational spectrum and the nature of the modes are very similar to those predicted for zero-temperature idealized sphere models and found in atomic and molecular glasses; there is a boson peak at low frequency that shifts to higher frequency as the system is compressed above the jamming transition.PACS numbers: 63. 63.50 Lm, 82.70 Dd Crystalline solids are all alike in their vibrational properties at low frequencies; every disordered solid is disordered in its own way. Disordered solids nonetheless exhibit common low-frequency vibrational properties that are completely unlike those of crystals, which are dominated by sound modes. Disordered atomic or molecular solids generically exhibit a "boson peak," where many more modes appear than expected for sound. The excess modes of the boson peak are believed to be responsible for the unusual behavior of the heat capacity and thermal conductivity at low-to-intermediate temperatures in disordered solids [2].It has been proposed that a zero-temperature jamming transition may provide a framework for understanding this unexpected commonality [3]. For frictionless, idealized spheres this jamming transition lies at the threshold of mechanical stability, known as the isostatic point [3,4]. As a result of this coincidence, the vibrational behavior of the marginally jammed solid at densities just above the jamming transition is fundamentally different from that of ordinary elastic solids [5][6][7][8]. A new class of lowfrequency vibrational modes arises because the system is at the threshold of mechanical stability [10]; these modes give rise to a divergent boson peak at zero frequency [5]. As the system is compressed beyond the jamming transition, the boson peak shrinks in height and shifts upwards in frequency [5]. Generalizations of the idealized sphere model suggest that the boson peaks of a wide class of disordered solids may arise from proximity to the jamming transition [9][10][11][12]. Moreover, the jamming scenario predicts that systems with larger constituents such as colloids should also have boson peaks.Colloidal glasses offer signal advantages over atomic or molecular disordered solids because colloids can be tracked by video microscopy. Vibrational behavior has been explored in hard-sphere colloids [13] and vibrated granular packings [14], but difficulties with statistics [13] or micro-cracks [14] were encountered. In contrast, we use deformable, thermosensitive hydrogel particles to tune the packing fraction in situ. Our experiments show unambiguously that the commonality in vibrational properties observed in atomic and molecular glas...

We probe non-equilibrium properties of an active bacterial bath through measurements of correlations of passive tracer particles and the response function of a driven, optically trapped tracer. These measurements demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. In some cases, we observe $1/\sqrt{\omega}$ scaling in the noise spectrum which we show can be derived from a theoretical model incorporating coupled stress, orientation, and concentration fluctuations of the bacteria.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let